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86                             POTASSIUM HYDROXIDE

in loo c.c, of water and treated with an excess of caustic potash and then,
dropjby drop and with shaking, with ferrous sulphate solution until a black
precipitate appears ; the volume is then made up to 500 c.c., the liquid
filtered and the ferrocyanide in 25 c.c, of the filtrate ( = o-i gram of sub-
stance) determined as indicated under Potassium Ferrocyanide (5). From
the quantity and titre of the permanganate used the ferricyanide is calcu-
lated ; Fe X 11-769 = K6Fea(CN)12.

If the red prussiatc contains ferroeyanitlc, the, latter is determined
directly prior to reduction and allowance then made for it in the; above

POTASSIUM FERROGYANIDE (Yellow Prussiate of Potash)

K4Fc(CN), + 3Ha(.) *„ 422-4

Lemon-yellow crystals soluble in water. The usual impurities may
contain small quantities of carbonates, sulphate's, chlorides and soda. The
tests are as follows :

1. Carbonates.......-The powdered salt, is moistened with dilute hydro-
chloric acid to see if effervescence occurs.

"   2. Sulphates.—The I ; 20 solution, acidified wilh hydrochloric add,
is tested with barium chloride.

3.  Chlorides.—I gram is fused with 2 grams of nitre in a porcelain
crucible and the mass dissolved in water, acidified with nitric add and
tested with silver nitrate.

4.  Soda.—The solution of 1-2 grains in 100 c.c. of water is addilied
with a little hydrochloric acid and treated with a slight; excess of ferric
chloride, the liquid being littered and the excess of iron in Hie filtrate pre-
cipitated with ammonia.   The solution is again littered, the filtrate being
concentrated to small volume and tested for sodium by means of potassium

5.  Quantitative Determination.-......-10 grams are dissolved in water

to I litre, 10 c.c. (== o-i gram of substance) being diluted in a porcelain
dish with about 250 c.c. of water, acidified with dilute sulphuric acid and
titrated with permanganate.    The ferrocyanide is then calculated from the
titre of the permanganate :   Fo x 7-56 •......; K4Fe(rN),,, ;vU20.

Commercial yellow prussiate is usually moderately pure, containing at kuust
99% of K4Fc(CN)fl, 3HaO.


sC-i (56)

This is sold in lumps, scales, sticks (ingots) or powder, or in solution
and in various degrees of purity, such as Cumtic putaxh puriatt. or by baryta,
prepared from the sulphate and from baryta, Caustic polaxh pure by alcohol,
Caustic potash pure by lime, and Ctudc. caustic potash. The impurities it
contains vary in nature (especially carbonates, sulphates, chlorides, nitrates,oloration or